Serial Communications (16C452)
The ZT 8809A COM1 and COM2 are shipped configured as DCE.
However, if the opposite configuration is desired for either of these
ports, see Appendix A for the required jumper configurations.
If you prefer to use only a "three-wire" serial interface (that is, TXD,
RXD, and ground), the ZT 8809A can be used without the RTS and
CTS lines. Both serial devices appear to be always ready to transmit
and receive.
System software is more complicated this way because both serial
devices have to keep a sharp lookout for transmitter and receiver
buffer activity. This is best accomplished on the ZT 8809A by using
interrupts. Details on the different aspects of interrupt usage are
discussed later in the serial interrupt register sections of this chapter.
Functionally, whenever an incoming data stream fills the receiver
buffer, an interrupt should be generated, telling the CPU to hurry up
and read the receiver buffer for incoming data. The transmit process is
less critical, but we recommend that the processor be interrupted
when the transmitter buffer is ready to be loaded again by the CPU;
that is, when the previous transmission is complete.
If the software for handling the serial control lines has already been
designed and you want to implement the three-wire serial link, the
ZT 8809A can be jumpered to loop RTS back to CTS, and DTR to
DSR. This is done by wire-wrapping these signals to each other in the
area provided (see Figure 8-2).
8-6
